There are a variety of optical systems in which high precision mirrors are needed. Examples include ultraviolet lithography condenser mirrors, reflective laser scanner mirrors, exo-atmospheric reflecting optics, systems in which low scattering is needed for visible radiation, and a variety of other applications.
One existing technique for making precision metal mirrors is to take a substrate of an aluminum alloy, such as that commonly known in the art as aluminum 6061-T6, and to carry out single point diamond turning (DPT) of a surface on the substrate, which then serves as the reflective surface. This alloy is lightweight, is easily machined by DPT, and has good long-term stability. Unfortunately, however, this aluminum alloy contains alloy elements such as zinc, chromium and iron, which leave defects or artifacts after DPT that effectively limit the surface finish achievable with DPT to approximately 80 Angstroms RMS. A surface finish of this quality provides adequately low scatter for many applications in which the reflected radiation of interest has a relatively long wavelength, such as a wavelength greater than 3 microns. At shorter wavelengths, however, such as the wavelength of visible radiation, a surface finish of 80 Angstroms RMS produces a level of scatter which is too high for many applications.
In attempt to address this problem, one prior technique took the approach of plating electroless nickel over the DPT surface on the Aluminum 6061-T6 substrate, subjecting this nickel layer to DPT, and then post-polishing the nickel layer to improve the surface finish. This alternative approach had several different disadvantages.
In particular, for all such mirrors, there is an inherent mismatch between the coefficient of thermal expansion (CTE) for the aluminum alloy substrate and the CTE for the plated nickel layer. Consequently, and due to the thickness of the nickel layer, there can be a bimetallic effect between the substrate and nickel layer, which causes bending of the mirror surface across the operational temperature range of the mirror. Such bending is undesirable in the context of a high precision mirror surface, because it changes optical characteristics of the mirror surface. In order to keep such bimetallic bending to an acceptable level over normal temperature variations, the plated nickel has to be a thin and uniform layer with a thickness of approximately 0.0005 inch to approximately 0.001 inch. In order to achieve this thin and uniform layer, the above-mentioned DPT operation was needed, in addition to the DPT operation for the aluminum. The need for this second DPT operation added cost and cycle time to the fabrication process.
An alternative technique uses electro-deposited high purity aluminum plating instead of the nickel plating, in order to avoid the thermal mismatch. This process creates a hard aluminum oxide (sapphire) outer layer on the electro-deposited aluminum layer, and this oxide layer damages the diamond tool during cutting. In addition, this high purity aluminum is very soft, and tends to build up on the diamond tool during cutting, which makes the DPT operation difficult. Also, the soft aluminum surface is easily scratched, and difficult to clean. Using this approach, the DPT surface finish is improved to around 40 Angstroms RMS, but the grooves from the DPT are still present, and create undesirable scatter at visible wavelengths.
One final method for improving surface finishes on 6061-T6 aluminum mirrors is to polish the aluminum. This is difficult because of the softness of the aluminum, although some techniques have demonstrated surface finishes of 10 Angstroms RMS. However, bi-directional reflective distribution function (BRDF) scatter testing shows that the resulting aspheric surface on the polished 6061-T6 aluminum layer effectively performs like a 60 Angstroms RMS surface finish, because the surface peak-to-valley variations remain very high as a result of impurities. Obviously, performing comparable to a 60 Angstroms RMS surface finish is not a significant improvement over an 80 Angstroms RMS surface finish, especially in view of the added difficulty and cost involved.